Corrosion-resistant nickel-chromium base alloy

ABSTRACT

A nickel-chromium base alloy adapted for turbine blade manufacture contains, in addition to nickel and chromium, cobalt and correlated amounts of niobium, titanium and aluminum, as well as carbon and boron. Other elements such as zirconium, rare earth metal and yttrium can be present.

United States Patent Inventors Priority Paul Isidore Fontalne Shirley,Solihull;

Michael John Fleetwood, Berkhamsted; Harry Lewis, Northfield,Birmingham, all of England June 5, 1969 Nov. 2, 197 l The InternationalNickel Company, Inc. New York, NY.

June 11, 1968 Great Britain CORROSION-RESISTANT NICKEL-CHROMIUM BASEALLOY 1 1 Claims, No Drawings [52] U.S.Cl 75/17l,

l48/32.5, 148/162 [51] Int. Cl C22c 19/00 [50] Field ofSearch 75/171,170; 148/32, 325,162

[56] References Cited UNITED STATES PATENTS 3,408,179 10/1968 Lewisetal75/171 3,466,171 9/1969 Fletcher et a1 75/171 Primary Examiner-Richard0. Dean Attorney-Maurice L. Pine] ABSTRACT: A nickel-chromium base alloyadapted for turbine blade manufacture contains, in addition to nickeland chromium, cobalt and correlated amounts of niobium, titanium andaluminum, as well as carbon and boron. Other elements such as zirconium,rare earth metal and yttrium can be present.

CORROSION-RESISTANT NlCKEL-CHROMIUM BASE ALLOY As those skilled in theart are aware, over the last score of years research efforts have beencontinuous and relentless in the quest for new and improved materialscapable of delivering improved performance at elevated temperatures.And, prompting this developmental process have been the ever-increasingand stringent demands imposed by commercial operating conditions. Thishas been true, for example, in respect of components for gas turbineengines, rotor. blades. beingillustrative. In this regard and in respectof aircraft gas turbines, the nickel-chromium base alloys have beenextensively used for such applications, alloys containing from topercent chromium together with certain amounts of titanium, aluminum andother constituents having been found generally acceptable.

For combating the more severe corrosive conditions encountered. withland-based turbines, a factor arising from the use of cheaper and lesspure fuel than aviation kerosene, alloys having greater corrosionresistance are necessary, and the use of higher chromium contents, interalia, has been advanced. This aspect has required a compositionalbalance to minimize loss in stress-rupture and creep characteristics. Itmight also be added, that even aircraft gas turbines operating in marineenvironments have to withstand severe corrosion attack as a result ofingestion of salt spray. And similar conditions are encountered in gasturbines used in ships and hovercraft.

To meet the requirements of both aircraft and land-based gas turbines,there are described and claimed in British Pat. No. 959,509 alloyscontaining 27 to percent chromium, 1.2 to 2.5 percent titanium, 0.5 to1.1 percent aluminum, the total titanium and aluminum being from 2.0 to3.2 percent, 0.01 to 0.1 percent carbon, 0.001 to 0.01 percent boron,0.01 to 0.1 percent zirconium and up to 1 percent silicon, the balancebeing essentially nickel. Such alloys generally have stress-rupturelives in the range of to 140 hours when tested under a stress of 17tonf/in. at 750 C. in the wrought condition after solution-heating andaging. As indicated in the subsequently issued British Pat. No.1,040,797, the stress-rupture life under these test conditions can beincreased to 200 to 300 hours by the simultaneous addition to the alloysof 12 to 30 percent cobalt and 1 to 7 percent molybdenum, respectively.

Such attributes notwithstanding, it was subsequently found, however,that embrittlement upon prolonged heating at high temperature ensued.This dilemma, it was determined, was largely brought about by thepresence of molybdenum, a constituent used to afford high stress-rupturelife at elevated temperature. Therefore, the problem to which thepresent invention is addressed is overcoming this disadvantage withoutincurring significant loss in either corrosion resistance orstressrupture characteristics at high temperatures.

It has now been discovered that the foregoing objective can be achieved,indeed, stress-rupture strength can also be improved, with substantiallyor entirely molybdenum-free alloys that contain small amounts of niobiumand in which the contents of chromium, titanium, aluminum and niobiumare specially interrelated.

It is an object of the invention to provide nickel-base alloys which byvirtue of good stress-rupture characteristics resistance to corrosionand improved long term stability at elevated temperature are suitablefor use in the production .of such articles as rotor blades for gasturbines.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, the alloys contemplated in accordance herewithcontain (by weight) at least 27 percent chromium for goodhigh-temperature corrosion resistance but not more than about 31 percentin order to avoid risking undue embrittlement. Preferably the chromiumcontent is from 28 percent to 29.5 percenLFrom 10 to 25 percent cobaltstrengthens the alloys, and it is to advantage that from 15 to 22percent cobalt is present. The alloys are further strengthened by thecopresence of niobium, titanium and aluminum. in this connection,stressrupture strength falls off markedly at niobium contents less than0.2 percent and beneficially the niobium content is from 0.3 to 1.5percent. More than 2 percent niobium leads to embrittlement and loss ofimpact strength, and also to loss of stress-rupture strength andductility. (Tantalum may be introduced incidentally with the niobium inan amount up to about one-tenth of the niobium content. For the purposesof the present invention, such amounts of tantalum are to be regarded aspart of the niobium content.)

The sum of the titanium and aluminum contents must be from 2.25 to 4.5percent, for either above or below these limits the stress-rupturestrength falls off, and too much titanium and aluminum also renders thealloys susceptible to embrittlement on prolonged heating at hightemperatures. Advantageously, the sum of these constituents is from 3 to4 percent. It is to be also noted that stress-rupture strength alsodepends on the ratio of titanium to aluminum, and this. must be from 1:1to 4:1, and is preferably from 1.521 to 2.521. The best combination ofstrength and elongation in stress-rupture tests is shown by alloys inwhich this ratio is about 2: l.

The foregoing notwithstanding, even within the narrow ranges of niobium,titanium and aluminum, some of thealloys may embrittle on prolongedheating at elevated temperatures and in order to minimize or avoid thisit is necessary to correlate the percentages of chromium, titanium,aluminum and niobium such that the value of the expression (hereinafterreferred to as the A Factor) does not exceed 40.

Carbon is also of importance. If it is too low, stress-rupture strengthis reduced, while if to the excess the alleys become susceptible toembrittlement. Hence, the carbon content should be from 0,2 to 0.1percent, and is preferably from 0.04 to 0.08 percent.

Boron and to a lesser extent zirconium both improve the stress-rupturestrength of the alloys, and they must contain at least 0.002 but notmore than 0.01 percent boron. Zirconium may be present in, amounts up to0.6 percent but no particular advantage is found in using more than 0.1percent.

The resistance of the alloys to oxidation and scaling is improved by thepresence of rare earth metals, and one or more of these metals may beadded, for example, in the form of misch metal. Advantageously, from0.01 to 0.3 percent of rare earth metal, e.g., from about 0.03 to 0.08percent, is added. Yttrium too improves oxidation and scaling resistanceand also resistance to sulphidation. Accordingly, yttrium mayadvantageously be added in amounts from 0.2 to 2 percent, for example,from 0,5 to 1 percent.

Of the elements that may be present as impurities, silicon has adeleterious effect on corrosion resistance and should therefore be keptbelow 1 percent and preferably below 0.5 percent. Other impurities mayinclude manganese in amounts up to 1 percent and iron in amounts up to 2percent.

A particularly advantageous combination of properties is exhibited byalloys containing from about 0.04 to 0.06 percent carbon, 28 or 28.5 to29 percent chromium, 19 to 21 percent cobalt, 2.1 or 2.2 to 2.5 percenttitanium, 1.1 to 1.3 or 1.4 percent aluminum, 0.5 to 0.8 or 1 percentniobium, from 0.002 to 0.01 percent, e.g., 0.003 to 0.005 percent boron,boron, up to, say, 0.06 or 0.1 percent zirconium, a range of 0.04 to 0.1percent zirconium being quite satisfactory, up to 0.3 percent rare earthmetal, up to 1% yttrium and the balance, apart from impurities, beingessentially nickel.

To develop the full stress-rupture properties of the alloys in wroughtform they must be subjected to a heat treatment comprising solutionheating and subsequent aging. The solution treatment may compriseheating from 1 to 8 hours in the temperature range of 1,050 to 1,200 C.,and the alloys may then be aged by heating for 1 to 24 hours in thetemperature range of 800 to 950 C. An intermediate aging treatmentconsisting of heating for l to 16 hours at 800 to l,050 C. may beinterposed between the solution treatment and the final aging stages.The alloys may be cooled at any convenient rate after each heattreatment stage, e.g., by air-cooling (generally to As will be seen fromresults in table II, alloy D, which contained too little titanium andaluminum, had a low stress-rupture life in comparison with alloys 1 and2 according to the invention. Alloy 5, which contained too much niobiumand had room temperature) or by direct transfer from a furnace at one 5an A Factor greater than 40, had very low impact strength temperature toone at a lower temperature. after prolonged heating at 850 C. In markedcontrast, alloy 3,

For the purpose of giving those skilled in the art a better aphaving anA Factor value of less than 40 is in accordance preciation of theinvention the following illustrative data are herewith, exhibited bothgood stress-rupture properties and given. good impact resistance. AlloyF contained too much niobium,

A series of alloys within the invention, alloys 1-3, table I, 10 andalthough it had good stress-rupture life, its A" Factor were tested inthe form of specimens machined from forged was greater than 40 and itsimpact strength was low. lts stressbar that had been heat treated bysolution heating for 4 hours rupture elongation was also rather low.Alloy F is an example at l,l50 C., air-cooling, aging for 16 hours atl,050 C., airof an alloy which is excluded from the invention onlybecause cooling and finally aging for 16 hours at 850 C. and againairthe contents of niobium, titanium, aluminum and chromium cooling. Forpurposes of comparison, there is included some 15 are not interrelatedas required by the invention. of the best alloys (alloys A, B and C) inaccordance with the The resistance of alloys according to the inventionto corroaforementioned British Pat. No. 1,040,797. The test resultsresion by the combustion products of impure hydrocarbon fuels portedwere obtained using a test temperature of8l5 C. with and by marine saltswas assessed (table 111) by tests in which a stress of 12.5 tonf/inF.specimens were exposed to a molten mixture of 25 percent by TABLE IComposition, percent Stress rupture Elon- Life gation,

C Cr Mo Ti Al Nb Zr B (hours) percent Alloy I A (0. 04) (30) (20) (2)(1. 7) (0. 8) NJ}. (0. 05) (0. 003) 179 4. 6

B- 0. (30) (20) (2; (1. 7) (0. 8) N.a. (0. 05) (0. 003) 141 5. 7

C- (28) (20) (4 1. 7 0. 85 NJ}. 0. 05 0. 003 144 20. 2

3 28. l 20. 1 N.a. 2. 50 l. 40 1. 1 0. 06 0. 003 864 5. 6

Nora:

=Balance substantially all nickel. (#)=nominal.

N.a.=not added.

The necessity of correlating the respective percentages of weight ofsodium chloride and 75 percent sodium sulfate at chromium, titanium,aluminum and niobium to avoid embrit- 900 C. The corrosion damage wasevaluated by comparing tlement is reflected by the results ofstress-rupture and impact the weight of each specimen, after removingthe corrosion tests as given in table II. All the specimens weremachined products by cathodic descaling in molten sodium hydroxide, fromforged bar that had been solution treated for 4 hours at 40 with theinitial weight before exposure. The more resistant l,l50 C. andair-cooled. Alloys D, l, 2 and E were given the materials are those thatshow the least loss in weight. The tests double aging treatment used forthe tests in table I, while alwere performed in two ways: in test A,samples of each alloy loys 3, F and G were aged in a single stage byheating for 16 were half immersed in the salt mixture while heated inair hours at 850 C. and air-cooled. The tests were then perwhereas intest B samples of each alloy were heated in a vertiformed under the sameconditions as those in table I, but the 4 cal open-top furnace intowhich the salt mixture was continuspecimens used for the impact tests(Charpy V-notch were ously fed as a fine dispersion at a rate of5g./hour.

TABLE III Weight loss (mg/cm!) Test A Test B after Composition, percentafter 300 72 121 C Cr Co Ti Al Nb Zr 13 hours hours hours Alloy:

Balance substantially all nickel.

heated for a further 1,000 hours at 850 C b f testing (A1- 60 The dataset forth in table 111 indicate that the corrosion reloys D, E and F, adi ti t f 1 2 d 3, are id the i sistance of alloy 1 according to theinvention is comparable to vention.) that of alloy .1, which is acommercially available alloy having TABLE II Composition, percent Stressrupture Elon- A Life gatlon, Impact C Cr Co Nb Ti Al B Zr factor (hours)(percent) strength Balance substantially all nickel.

that of alloy H, which is a commercially available alloy of comparablestress-rupture strength but lower chromium content.

The alloys can be air melted, but to ensure the best creep propertiesthey are preferably melted and cast under vacuum. They can be readilyprocessed by conventional means such as extrusion, forging, or rolling.Although primarily intended for use in the wrought form as gas turbineblades, the subject alloys are suitable for use in other applicationswhere a combination of good stress-rupture strength and resistance tocorrosion is required, particularly for articles ad parts that aresubjected in use to stress at high temperatures while exposed to thecombustion products of impure hydrocarbon fuels or to salt or both. Theymay also be used to make cast articles and parts, which may be used withor without heat treatment.

As will be appreciated by those skilled in the art, the term balance orbalance essentially" usedin referring to the nickel content does notexclude the presence of small amounts of other elements, commonlypresent as incidental elements, e.g., deoxidizing and cleansingconstituents, and impurities ordinarily associated therewith in smallamounts which do no adversely affect the basic characteristics of thealloys.

Although the present invention has been described in conjunction withpreferred embodiments it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention. Such modifications and variations are consideredto be within the purview and scope of the invention and appended claims.

We claim:

1. An alloy consisting of about 27 to about 31 percent chromium, aboutto 25 percent cobalt, from 0.2 to 2 percent niobium, about 2.25 to about4.5 percent total of titanium and aluminum with the provisos that (a)the ratio of titanium to aluminum is from about 1:1 to about 4:1 and (b)the much lower stress-rupture strength and greatly superior to V valueof the A Factor as determined by the relationship5(%Nb)+4(%Ti+Al)+c/(%Cr) does not exceed about 40, about 0.02 to 0.1percent carbon, about 0.002 to 0.01 percent boron, up to 0.6 percentzirconium, up to about 0.3 percent of rare earth metal, up to 2 percentyttrium and the balance essentially nickel.

2. An alloy in accordance with claim 1 containing 15 to 22 percentcobalt.

3. An alloy in accordance with claim 1 containing 0.3 to 1.5 percentniobium.

4. An alloy in accordance with claim 1 containing about 3 to 4 percenttotal of titanium plus aluminum.

5. An alloy in accordance with claim 1 in which the ratio of titanium toaluminum is from 1.5 :l to 2.5: l.

6. An alloy in accordance with claim 1 containing about 0.04 to 0.8percent carbon.

7. An alloy in accordance with claim 1 containing about 0.01 to 0.3percent rare earth metal.

8. An alloy in accordance with claim 1 containing 0.03 to 0.8 percentrare earth metal.

9. An alloy in accordance with claim 1 containing about 0.2 to 2 percentyttrium.

10. An alloy in accordance with claim 1 containing about 0.5 to 1percent yttrium.

11. An alloy in accordance with claim 1 containing about 28 to about 29percent chromium, about 19 to about 21 percent cobalt, about 0.5 toabout 1 percent niobium, about 2.1 to about 2.5 percent titanium, about1.1 to about 1.4 percent aluminum, about 0.04 to about 0.06 percentcarbon, about 0.002 to about 0.01 percent boron, about 0.04 to about 0.1percent zirconium, up to about 0.3 percent rare earth metal, up to about1 percent yttrium, and the balance essentially nickel.

2. An alloy in accordance with claim 1 containing 15 to 22 percent cobalt.
 3. An alloy in accordance with claim 1 containing 0.3 to 1.5 percent niobium.
 4. An alloy in accordance with claim 1 containing about 3 to 4 percent total of titanium plus aluminum.
 5. An alloy in accordance with claim 1 in which the ratio of titanium to aluminum is from 1.5:1 to 2.5:1.
 6. An alloy in accordance with claim 1 containing about 0.04 to 0.8 percent carbon.
 7. An alloy in accordance with claim 1 containing about 0.01 to 0.3 percent rare earth metal.
 8. An alloy in accordance with claim 1 containing 0.03 to 0.8 percent rare earth metal.
 9. An alloy in accordance with claim 1 containing about 0.2 to 2 percent yttrium.
 10. An alloy in accordance with claim 1 containing about 0.5 to 1 percent yttrium.
 11. An alloy in accordance with claim 1 containing about 28 to about 29 percent chromium, about 19 to about 21 percent cobalt, about 0.5 to about 1 percent niobium, about 2.1 to about 2.5 percent titanium, about 1.1 to about 1.4 percent aluminum, about 0.04 to about 0.06 percent carbon, about 0.002 to about 0.01 percent boron, about 0.04 to about 0.1 percent zirconium, up to about 0.3 percent rare earth metal, up to about 1 percent yttrium, and the balance essentially nickel. 